Aromatic polyester resins have excellent mechanical properties; however, their barrier properties to gases such as CO2 and oxygen are not sufficiently high. The barrier properties can be improved by mixing the polyester resin, in the molten state, with a polyxylylene amide, particularly poly-MXD-6, used in quantity of about 5 to 30% by weight.
The barrier properties with respect to oxygen and CO2 are further increased if the composition comprising the polyester resin and the polyxylylene amide is prepared by pre-mixing the polyester resin, in the molten state, with a dianhydride of a tetracarboxylic acid, particularly pyromellitic dianhydride (PMDA), and subsequently adding and mixing the polyamide with the molten polyester resin (EP-A-964031).
The addition of the polyxylylene amide either to the polyester resin premixed with the dianhydride or not containing the dianhydride, has the effect of reducing the transparency of the obtained articles.
The reduction in trasparency increases with the increase of the quantity of polyamide and of its molecular weight (WO-A-93/20147). Average numeral molecular weights of less than 15000 and amounts of polyamide less than 2 wt % are needed to limit the increase of opacity.
A need therefore exists of. providing polyester resins added with significant quantities of polyxylylene amide, which combine satisfactory barrier properties to oxygen and CO2 with high transparency and colour characteristics.